The present invention relates generally to a device for measuring the air gap between a proximity sensor and its target and, more specifically, to a device for determining whether a proximity sensor in the door of an aircraft is installed correctly and functioning properly.
A proximity sensor is a well-known device that may be used in a system for detecting the presence of a metallic target within a threshold distance from the sensor. A proximity sensor has a complex impedance and is electrically equivalent to an inductor in series with a small resistor. Such proximity sensors are often referred to as "variable reluctance" devices because the presence of the metallic target in close proximity to a sensor inductively varies the sensor impedance. This variation in sensor impedance can be sensed by electronic circuitry, which can thus provide an indication of the presence or absence of the target in close proximity to the sensor. Because most such sensors have two leads for connecting the sensor to the electronic circuitry, they are known as "two-wire" sensors. Some proximity sensors, however, include a third lead that allows the electronic circuitry to compensate for the resistance of the wires connected to the other two leads. Such sensors are known as "three-wire" sensors.
Proximity sensors are commonly used aboard aircraft to detect whether a mechanical device, such as a cargo door, landing gear, control surface, or thrust reverser, is properly positioned. The electronic circuitry may be connected to a status light in the cockpit to provide the pilot with an indication of the position of the mechanical device.
To detect the position of a cargo door, for example, the target is installed inside the door and the sensor is installed in the door frame such that the sensor and target are adjacent and separated by a small air gap when the door is in the fully-closed position. This gap is known as the "rigging gap" and may be specified by the manufacturer of the sensor. Aircraft manufacturers installing a proximity sensor and maintenance personnel inspecting a proximity sensor must ensure that the gap is properly set to provide reliable position detection.
Cockpit status indicators may provide incorrect position indications if either the sensor itself or the electronic circuitry connected to the sensor, including the status light, fails. Sensors are susceptible to many types of failures. In one common failure mode, for example, the electrical resistance of the sensor increases. In addition to sensor failure, aircraft vibration or other physical movement may alter the positions of the target or sensor relative to one another, thereby changing the rigging gap and possibly preventing the sensor from detecting the target. Isolating the source of such sensor system problems to either the sensor itself or the associated sensor electronics can be a difficult and time consuming task.
Practitioners have developed methods for testing proximity sensors, such as measuring sensor resistance, to isolate the cause of sensor problems. However, the rigging gap is often difficult or impossible to measure because the sensor and target may be inaccessible when the mechanical structures in which they are installed are in close proximity, such as when a cargo door is in the closed position. Moreover, the target is sometimes completely embedded in the structure where it is not only inaccessible but also hidden from view.
Several methods are known that may be used to measure the gap where the sensor and target are accessible. In one such method, thin feeler gauges having known widths are wedged between the sensor and target. The sum of the widths of the gauges equals the rigging gap. As discussed above, this method is unsuitable for measuring the gap where insufficient room exists to allow the gauges to be inserted. For example, the shape of a cargo door may prevent anything from being inserted between the door and its surrounding frame.
In another such method, a wad of putty or modeling clay is placed on the sensor face while the target and sensor are separated, such as when a cargo door is open. When the cargo door is shut, the modeling clay is compacted to approximately the sensor target gap distance. The door is then opened and the compacted modeling clay is retrieved and measured with calipers. This method can provide a rough rigging gap measurement where insufficient room exists to wedge feeler gauges between the mechanical structures. However, this method is unsuitable for measuring a rigging gap between a sensor and a target installed in mechanical structures that slide relative to one another rather than swing on hinges. In such structures, feeler gauges or modeling clay inserted between the two portions may not only be difficult to align between the sensor and target but could easily be broken, deformed or otherwise rendered unretrievable by the sliding structures.
In addition to the methods discussed above, an electromechanical device for measuring the rigging gap of proximity sensors installed in aircraft has been produced by ELDEC Corporation of Lynnwood, Wash. and is described in "Operator's Manual with Illustrated Parts List," ELDEC Document No. 011-2851-104, Nov. 1, 1985. The ELDEC device has mounted within it a reference proximity sensor and a reference target. The reference sensor and target are separated by a gap, which an operator can manually adjust using a vernier screw adjustment that displays the gap in inches. The ELDEC device has electronic circuitry, including an indicator lamp, that is connected to the reference sensor and duplicates the aircraft electronics to which the sensor to be tested is connected.
To use the ELDEC device to measure the rigging gap of a sensor, an operator disconnects the sensor leads from the aircraft electronics and connects them to terminals on the ELDEC device. The device has a switch that connects the electronics in the device to either the sensor-under-test or the reference sensor. The operator manually sets the switch to connect the device electronics to the sensor-under test. The operator then manually adjusts a potentiometer on the device until the indicator lamp is activated. The operator then sets the switch to connect the device electronics to the reference sensor and adjusts the vernier screw until the indicator lamp is activated. At that time, the impedance of the sensor-under-test is approximately equal to the impedance of the reference sensor. Thus, if the reference sensor and the sensor-under-test are of the same type, i.e., they have identical impedances at any given distance from a target, the gap between the sensor-under-test and its target should at that time be equal to the gap between the reference sensor and its target, which the operator can read on the vernier display. However, because different types of sensors are commonly used, an operator must use a printed conversion table to convert the measured gap into a corresponding gap measurement for a sensor of the type under test.
Although the ELDEC device overcomes several of the problems involved in proximity sensor gap measurement, the ELDEC device does not test sensor operation. As a result, rigging gap measurements may be inaccurate because the sensor is not functioning properly. Moreover, using a conversion table developed from empirical data may introduce undesirable error into the gap measurement. Furthermore, contrary to the assumption underlying the use of the ELDEC device, the electronics in the ELDEC device may not be exactly identical to that in the aircraft.
A system that both tests sensor operation and precisely measures the rigging gap without dependence upon additional sources of error, such as reference sensors, would be highly desirable. These problems and deficiencies are clearly felt in the art and are solved by the present invention in the manner described below.